1. Field of the Invention
This invention relates generally to both a method and apparatus for the determination of a relative concentration of a first tissue chromophore with respect to a total concentration of a second, but related tissue chromophore. Specifically, the invention is a system for determining the concentration of oxyhemoglobin relative to the total concentration of hemoglobin within a blood-containing (blood perfused) tissue.
2. Description of the Related Art
Generally, when diagnosing the function of a body organ such as the cerebral tissues or the heart, the important parameters to measure are the oxygen quantity in the body organ and the organ's utilization of oxygen. Supplying body organs with a sufficient quantity of oxygen is also important for the growth of infants. An insufficiency of oxygen can affect many body organs.
Numerous apparatus and methods for examining the oxygen quantity in body organs readily and at various stages of illness are known in the art. For example, U.S. Pat. No. 4,281,645, issued Aug. 4, 1981 teaches the measurement of variations in oxygen quantity in body organs using apparatus in vivo to measure the absorption spectrum of near infrared light for the tissue of interest. In particular, the absorption is caused by the hemoglobin which is an oxygen-carrying medium in blood, and the cytochrome a,a.sub.3 which performs an oxidation-reduction reaction in cells. The '645 patent discloses and teaches the use of four separate near infrared light rays having distinct wavelengths different from one another to determine the variation of cerebral oxygen quantity. The apparatus and techniques taught by the '645 patent utilize the relationship "Abs=a*l*c", where "Abs" is an optical absorption value, "a" is the empirically determined absorption coefficient for a particular chromophore, i.e. hemoglobin, "l" is the optical pathlength over which the radiated light travels, and "c" is the density or concentration of the chromophore of interest being measured. The system of the '645 patent produces an output signal representing the difference in or ratio of absorption of the measuring and reference wavelengths by the organ or other corporeal portion of the body as a function of the state of the metabolic activity in vivo, which may be converted to a signal providing a substantially continuous measure of such activity.
Other related techniques and apparatus have also been developed and are known. For example, U.S. Pat. No. 4,805,623, issued Feb. 21, 1989, to Jobsis, and entitled Spectrophotometric Method For Quantitatively Determining The Concentration Of A Dilute Component In A Light Or Other Radiation-Scattering Environment, teaches quantitatively determining the concentration of a dilute component in either a clear or a strongly light-scattering environment which also contains a reference component of known concentration. The method uses a series of contemporaneous radiation-directing and measurement steps of radiation of selected varying wavelengths. Thus, the method taught by Jobsis requires prior knowledge of the concentration of a reference component in order to determine the unknown concentration of the component of interest.
U.S. Pat. No. 5,139,025, issued Aug. 18, 1992, to Lewis et al., and entitled Method And Apparatus For In Vivo Optical Spectroscopic Examination, teaches clinical evaluation of biological matter, in particular human anatomy, examined in situ and in vivo, by selective spectral light transmissivity. The method and apparatus disclosed by Lewis et al. requires knowledge of conditioning factors in quantifying the resulting light-reception data, the conditioning factors consisting of relative geometrical locations and spacing of the light receivers and the nominal optical distance, and particularly the difference between the optical distance between the location of the near receptor, or receiver, and that of the far receptor or receiver.
Other methods and apparatus known in the art generally require knowledge of or precise measurement of the optical pathlength and/or empirical calculations of absorption coefficients for a component to be measured, or otherwise are useful only in providing trend data relating to the component of interest to be measured. One such example, U. S. Pat. No. 5,482,034, issued Jan. 9, 1996, to Lewis et al., and entitled Method And Apparatus For Spectrophotometric Cerebral Oximetry And The Like, teaches the processing of signals representative of the radiation detected by first and second receivers, to obtain data which particularly characterizes selected attributes of the measured substance within the particular internal region. The method and apparatus taught by the '034 patent requires precise placement of the first and second receivers with respect to one another.
Another technique is disclosed by S. J. Matcher and C. E. Cooper in Absolute quantification of deoxyhemoglobin concentration in tissue near infrared spectroscopy, Phys. Med. Biol. 39 (1994), 1295-1312. Matcher et al. teaches a method, using a suitable multi-wavelength NIR spectrometer to obtain an in vivo second-differential spectrum, and applies multi-linear regression to fit the acquired spectrum with the reference second-differential spectra of hemoglobin (Hb) and water (H.sub.2 O). The ratio of Hb! to H.sub.2 O! is then multiplied by the assumed concentration of water in the tissue to yield Hb!.
In view of the foregoing, a need in the art still exists for a method and apparatus suitable for providing accurate measurements relating to particular chromophores, but which is substantially robust to changes in optical pathlength and/or changes in total concentrations of specific chromophores to be measured. Preferably, such a method and apparatus will yield relative concentrations of a first chromophore, i.e. oxyhemoglobin with respect to the total concentration of a different, but related chromophore, i.e. hemoglobin, regardless of the total concentration of the related chromophore, and substantially unaffected by probe placement. The measurement apparatus should also be insensitive to the effects of scattering and interfering spectral contributors, e.g. melanin and bilirubin will not effect the accuracy of measurements.
Systems which measure the tissue chromophore of % oxyhemoglobin relative to total hemoglobin (StO.sub.2) are known in the art and commercially available. For example, systems of this type are available from ISS of Champaign, Ill. and Somanetics of Troy, Mich. Specifically, the ISS device measures absolute tissue absorbance by correlating the phase shift between the send and receive signals of modulated monochromatic light at multiple wavelengths. This methodology allows for direct measurement of optical pathlength and measurement of tissue chromophores using absorbance measurements. The ISS device also correlates absolute absorbance to StO.sub.2 using pure oxyhemoglobin and deoxyhemoglobin absorption coefficients. The absorption coefficients for mixtures of these two chromophores (values other than 0% and 100% StO.sub.2) are assumed to be constant within the measurement range (Beer-Lambert Law). What is still needed is a method and apparatus that does not require measurement of optical pathlength, has reduced complexity, and eliminates pathlength sensitivity, all while offering more accurate measurements between 0% and 100% StO.sub.2.
The Somanetics device measures tissue absorption values at two wavelengths within two different depths (two distinct send/receive spacings). The four absorption value measurements are empirically calibrated to StO.sub.2 units using a brain tissue model. Calibration spectra is obtained from human forehead measurements at conditions where the blood saturation of the jugular vein is also measured. The calibration spectra is assigned StO.sub.2 values calibrated from a weighted average of arterial and jugular vein measurements (field saturation). However, a field saturation measurement being an approximate estimate of tissue StO.sub.2 does not always correlate with true tissue readings. What is still needed is a calibration set in which the spectral measurements are obtained at accurately known values. The Somanetics device is subject to scattering variations which can induce significant errors within the respective measurements.